HVDC (high-voltage direct current) electrical power transmission uses direct current for the transmission of electrical power. This is an alternative to alternating current electrical power transmission which is more common. There are a number of benefits to using HVDC electrical power transmission. HVDC is particularly useful for power transmission over long distances and/or interconnecting alternating current (AC) networks that operate at different frequencies.
To date most HVDC transmission systems have been based on line commutated converters (LCCs), for example such as a six-pulse bridge converter using thyristor valves. LCCs use elements such as thyristors that can be turned on by appropriate trigger signals and remain conducting as long as they are forward biased.
Increasingly however voltage source converters (VSCs) are being proposed for use in HVDC transmission. VSCs use switching elements such as insulated-gate bipolar transistors (IGBTs) that can be controllably turned on and turned off independently of any connected AC system. VSCs are thus sometime referred to as self-commutating converters.
Various designs of VSC are known. Typically each VSC will have a phase limb for each AC phase, with each phase limb having two converter arms connecting the relevant AC terminal to respective high and low DC terminals. Each converter arm comprises an apparatus, often referred to as a valve, for selectively connecting the AC terminal to the relevant DC terminal.
In one form of known VSC, often referred to as a six pulse bridge or as a two-level converter, the valve of each converter arm comprises a set of series connected switching elements, typically IGBTs, each IGBT connected with an antiparallel diode. The IGBTs of the valve are switched together to connect or disconnect the relevant AC and DC terminals, with the valves of a given phase limb being switched in antiphase. By using a pulse width modulated (PWM) type switching scheme for each arm, conversion between AC and DC voltage can be achieved. In another form of VSC referred to as a modular multilevel converter (MMC) each converter arm comprises a plurality of series connected cells that each have an energy storage element such as a capacitor that can be selectively connected in series between the relevant AC and DC terminals or bypassed. By using a relatively large number of cells and timing the switching appropriately the valve can synthesise a stepped waveform that approximates to a sine wave, to convert from DC to AC or vice versa with low levels of harmonic distortion. In a further type of converter referred to as an Alternate-Arm-Converter (AAC) a plurality of series connected cells is connected in each converter arm for providing a stepped voltage waveform as described for the MMC type converter but each converter arm also comprises an arm switch, referred to as a director switch and each converter arm is turned off for at least part of the AC cycle.